Polyethylene microporous films are used as microfiltration membranes, separators for batteries, separators for condensers, etc. Among these uses, when polyethylene microporous films are used as battery separators, especially as separators for lithium ion batteries, they are required to have a so-called "fuse effect" which is a property that when the insides of the batteries are overheated, the separator becomes molten and forms a coat which covers the electrode to break the current, thereby securing the safety of batteries. In addition, general properties such as mechanical strength and permeability are necessary.
It is known that the fuse temperature of polyethylene microporous films, namely, the temperature at which the fuse effect is developed, is about 130-150.degree. C. Even if the inside of battery is overheated for some reason, when the temperature reaches the fuse temperature, the microporous film melts to form a coating which covers the electrodes, and, hence, the ionic flow is interrupted to stop the battery reaction. However, when the temperature very abruptly rises, the temperature inside the battery further rises even after the fuse occurs, and consequently said coat is broken, which causes a short in the battery. Separators having such a high short resistance, which can surely continue to break the current even under the above-mentioned circumstances, are in demand.
The object of the present invention is to provide a polyethylene microporous film excellent in mechanical strength, permeability and productivity, and high in short resistance.
As a result of intensive research conducted by the inventors, it has been found that a polyethylene microporous film comprising a specific high-density polyethylene or linear copolymer polyethylene has a high short resistance, and the present invention has been accomplished.